Recent advances on the emulsifying properties of dietary polysaccharides

Emulsion, a disperse system, generally consists of two immiscible liquids, where one of the liquids (dispersed phase) is dispersed as droplets in the other liquid (continuous phase). Taking emulsion as delivery system is a great strategy for enhancing the stability and bioavailability of bioactivity substances (Cao, et al., 2021; Jagtiani, 2021; Lu et al., 2016). Thus, emulsion system is widely used in food, pharmaceutical, and cosmetic industry. In particular, to enhance the flavor and taste of food, emulsion is also used in some common foods, such as mayonnaise, cream, and material for three-dimensional food printing (Figure 1). Emulsifier plays a key role in the formation of emulsion system. The most common emulsifiers contain small molecule surfactant, natural amphiphilic macromolecule, solid particle, and auxiliary emulsifier (Amiri-Rigi et al., 2023). Among them, amphiphilic polysaccharides, such as pectin, gum arabic, and galactomannans, are important members of the natural amphiphilic macromolecule, and have been utilized as food-grade emulsifiers (Feng, et al., 2023; Niu, Hou, et al., 2022). Compared with protein, the hydrated layer formed by polysaccharides possess relatively higher steric hindrance which improves the emulsion stability (Lin, et al., 2020). Furthermore, the low digestibility of polysaccharide in digestive tract will result in delaying release rate of the bioactivities (Anal et al., 2019). In view of the advantage and importance of amphiphilic polysaccharides, a growing number of studies focus on the discovery of natural polysaccharides which possess the ability to stabilize oil-water interface. As shown in Figure 2, the number of publications centered on “polysaccharide and emulsion” (Indexed by WOS) gradually increased since 2011 and rapidly increased in the last 3 years (from 2019 to 2021). This review highlights on recent advances in the emulsifying properties of polysaccharides, furtherly the structure–activity relationship, influencing factors, and improvement technologies.

The emulsifying properties of polysaccharides contain emulsifying activity and emulsifying stability. Emulsifying activity refers to the ability of polysaccharides to absorb on the oil-water interface and shape interfacial film. It presents as the droplet size of the emulsion stabilized by polysaccharides at critical concentration. In the case of emulsifying stability, it is reflected by the ability of interfacial film shaped by poysaccharides for preventing the aggregation of oil droplets and maintaining the uniform texture of emulsion during storage and process. The emulsifying properties of polysaccharides could be evaluated from several aspects as the followings.

The surface hydrophobicity index and interfacial tension are the most popular indirect indexes for forecasting the interfacial activity of polysaccharide (Chen, et al., 2019; Ravera, et al., 2021). Generally, the higher of the surface hydrophobicity index means the higher activity of polysaccharides to absorb onto the oil surface, and the lower interfacial tension indicates the higher ability of the polysaccharides to stabilize the oil−water interface. Besides, the turbidimetric method, containing the emulsifying activity index and emulsifying stability index, is a classic method for evaluating the emulsifying properties of polysaccharide methods (Yan et al., 2021). The advantage of this method is easy to execute, but the defect of this method is fail to determine the oil droplet size and distribution which are the most important data for study the emulsion system. Fortunately, with the development of science and technology, the instruments based on the laser diffraction and dynamic light scattering technology could be used to observe the droplet size and distribution of the emulsion stabilized by polysaccharides (Lin, Guo, et al., 2020; Lin, Yu, et al., 2020; Zhang et al., 2021). As a result, the droplet size and distribution of emulsion become the most popular and direct index to evaluate the emulsifying properties of polysaccharides. Furthermore, some optics and imaging technologies, such as optical microscope, transmission electron microscope, atomic force microscope, and confocal scanning laser microscope, could use to observe the microscopic morphology of droplets and calculate the droplets size by the corresponding ruler (Ai et al., 2019; Ho et al., 2022; Li et al., 2019). Notably, microscopic morphology is an important evidence to distinguish the coalescence, aggregation, and flocculation of emulsion droplets, thereby studying the emulsifying stability of polysaccharides (Ai, Meng, et al., 2022; Ai, Zhao, et al., 2022).

Recently studies show that the emulsifying properties of polysaccharides could be influenced by several structural elements, including esterified group, molecular weight, protein moiety, phenolic acids, and charged groups (e.g., amino group, carboxyl group, and sulfate group) (Figure 3). Among the above structural elements, the esterified group, protein moiety, and phenolic acids are regarded as hydrophobic anchors which absorbed onto the oil surface, and play vital roles of the emulsifying activity of polysaccharides (Ai et al., 2020). The molecular weight of polysaccharides is related to the viscosity and steric hindrance, which significantly influences the emulsion stability. Besides, the carboxyl group would provide electrostatic repulsion to reduce the close up of oil droplets and enhance the stability of emulsion system (Lin et al., 2021). In particular, the molecular configuration of polysaccharides is a special factor. It would not influence the emulsifying properties directly, but influence the exposure of the hydrophobic groups of polysaccharides, thereby effecting the efficiency and difficulty of hydrophobic group absorbing to the oil surface, as a result, the emulsifying activity of the polysaccharides could be influenced (Ai, Meng, et al., 2022; Ai, Zhao, et al., 2022; Jiang et al., 2020; Matsuyama, et al., 2021). Furthermore, the molecular configuration of polysaccharides also influences the thickness of the interfacial film which contributes to the stability of the emulsion system (Lin, Guo, et al., 2020; Lin, Yu, et al., 2020).

The external elements for influencing the emulsifying properties of polysaccharides mainly includes oil phase content, oil type, polysaccharide usage, energy input, pH value, salts, temperature, gravity, and so forth (Figure 4). The oil phase content/type and polysaccharide usage mainly influence the adsorption density of the hydrophobic anchor on the interface, thereby effecting the droplet size (Du, et al., 2022; Ma, et al., 2019; Zhu, et al., 2019). In addition, oil and polysaccharide are the main contributors of the viscosity of emulsion, which is highly related to the emulsion stability (Ai et al., 2019; Shao, et al., 2020). The most common energy input modes for the emulsion preparation include high speed shear, ultrasonic, high-pressure homogenization, and high-pressure microfluidization (Chen et al., 2018). The different emulsion preparation methods would influence the efficiency of polysaccharides to absorb onto oil surface, what's more, some violent energy input mode, such as high-pressure and high power ultrasonic, would result in the change of molecule chain of polysaccharides (Benchamas et al., 2020; Raoufi et al., 2019). Generally, the efficiency order of the above energy input mode is: high-pressure microfluidization > high-pressure homogenization > ultrasonic > high speed shear. Notably, the ultrasonic method is not applicable in oil-water systems with high viscosity due to the poor transmission efficiency. The effect of pH on the emulsifying properties of polysaccharides is mainly acted on the charged group (Ai et al., 2019; Xiong, et al., 2020). Obviously, the ionization degree of charged group will result in the change of intramolecular or intermolecular force, thereby changing the molecular configuration. Therefore, the optimal emulsifying properties of polysaccharides should be performed in a suitable pH value. In the case of salts, it could screen the electrostatic charge of polysaccharides in aqueous solution to reduce the electrostatic repulsion (Xu et al., 2017). Moreover, the multivalent cations ionized from salts, such as calcium, can shape as “calcium bridge” and electrostatically combine with multiple negatively charged polysaccharides, thus resulting in the aggregation of polysaccharides and changing the emulsifying properties of polysaccharides (Ai et al., 2020). Besides, settling or floating will be emerged in emulsion system due to the effect of gravity (Niu, Wang, et al., 2022). According to the “Stokes Law,” the settling or floating can be easier occurred in the emulsion system with high initial droplet size (Xu, et al., 2020). It suggests that when the emulsifying activity is effected which results in the change of initial droplet size of emulsion, the emulsion stability will also be influenced.

Natural polysaccharides are hard to meet the requirement for preparing specific emulsion system for nutrition delivering. Based on the above external and structural elements mentioned in Sections 3 and 4, researchers have discovered many effective methods to improve the emulsifying properties of polysaccharides (Figure 5). Adding exogenous proteins is an efficiency strategy to improve the emulsifying properties of polysaccharides. A common method for constructing polysaccaride-protein complex is based on the charged groups of polysaccharides and proteins, such as amino, sulfate, carboxyl, and so forth (Zhao, et al., 2020). These polymers possess varied ionization properties under different environmental conditions to form electrostatic complex. Furthermore, exogenous proteins could also be covalently linked with polysaccharides by using laccase, Maillard reaction, genipin crosslinking reaction, and so forth. (Ai, Meng, et al., 2022; Ai, Zhao, et al., 2022; Li & Karboune, 2021). The covalent linkage between protein and polysaccharide is more stable than electrostatic bonding, but the degree of grafting is hard to control, and the grafting sites is random. As a result, the unpurified covalent complex usually contains free protein, free polysaccharide, and various covalent complex with different reaction degree. Thus, the electrostatic interaction between the free protein and polysaccharide should be considered when the covalent complex is used as emulsifier. Beside, previous studies have demonstrated that some phenolic acids, like ferulic acid, gallic acid, and quercetin, could be linked with polysaccharide by covalent bonding as hydrophobic anchor (Liu et al., 2020). In addition, esterification of carboxyl groups on polysaccharides or acylation of hydroxyl groups of polysaccharides is also a good choice. For example, the emulsifying activity of chemically modified starch is closely related to esterification groups (Hadi, et al., 2020). In conclusion, the improvement of the emulsifying properties of polysaccharides is usually based on the following aspects: firstly, the emulsifying activity of polysaccharide could be improved by increasing the number of hydrophobic anchors for enhancing the adsorption capacity of polysaccharides on the surface of oil droplets; secondly, the emulsifying stability of polysaccharides could be enhanced by increasing the molecular weight of polysaccharides for enhancing the steric hindrance among oil droplet; thirdly, the ionic and pH sensitivity of polysaccharides could be reduced by modifying the functional groups of polysaccharides.

The complicated components of food system lead to high requirements for the emulsifiers which are safety and can maintain good emulsifying properties during food processing. As potential natural emulsifiers, the amphiphilic polysaccharides are also met the challenges from food components and processing, such as the electrostatic interaction with salts and protein, the hydrogen bonding interaction with other polymers, the hydrophobic interaction with polyphenol, thermal denaturation by heating, and so forth. Although the general regulations of the effects of external factors on the emulsifying properties of polysaccharides have seemed to be already concluded certainly, the widely different molecular structure of polysaccharide from various resource which results in the wide variations of the emulsifying properties of polysaccharide. It means that if a novel amphiphilic polysaccharide which is proposed to use as food grade emulsifier, the physicochemical characteristics of this polysaccharide should be completely evaluated. Besides, the emulsifying properties changes of polysaccharides in food processing (thermal or non-thermal) and its molecular mechanism are rarely investigated. Therefore, it is undoubtedly an arduous and long-term task for develop nature polysaccharide as food grade emulsifiers. Currently, the emulsion systems which prepared using polysaccharide as emulsifier are applied in the delivery of bioactives for investigating its mechanism of protection, release, digestion, absorption, and transportation. It means that a complete upstream and downstream research system could be shaped in the structural characterization of polysaccharide, interfacial behavior of polysaccharide, delivery characteristics of the emulsion stabilized by polysaccharide, digestive characteristics, and the metabolism kinetics and the bioavailability of loaded bioactives. However, the correlations and change laws between two different links are still critical problems to be solved in current. After solving these problem, polysaccharides with specific properties (natural or modified) can be used for constructing the desired emulsion system which could deliver optimal amount of bioactives, and possess desired ability to protect the activity of loaded bioactives. Even more, it is possible to use a suitable polysaccharide as an emulsifier to construct a proper emulsion system, so that the encapsulated bioactives could be metabolized in human body as expected.